In the semiconductor industry, different bonding technologies have already been used for several years in order to connect substrates to one another. The connecting process is called bonding. A rough distinction is made between the temporary bonding method and the permanent bonding method.
In the temporary bonding method, a product substrate is bonded to a carrier substrate in such a way that after processing, it can be detached again. Using the temporary bonding method, it is possible to stabilize a product substrate mechanically. The mechanical stabilization ensures that the product substrate can be handled without curving, deforming, or breaking. Stabilizations by carrier substrates are primarily necessary during and after a back-thinning process. A back-thinning process makes it possible to reduce the product substrate thickness to a few micrometers.
In the permanent bonding method, two substrates are bonded to one another continuously, i.e., permanently. The permanent bonding of two substrates also makes it possible to produce multi-layer structures. These multi-layer structures can be comprised of the same or different materials. Different permanent bonding methods exist.
The permanent bonding method of the anodic bonding is used in order to connect ion-containing substrates permanently to one another. In most cases, one of the two substrates is a glass substrate. The second substrate is preferably a silicon substrate. In the method, an electric field is applied along the two substrates that are to be bonded to one another. The electric field is produced between two electrodes, which preferably bring the two surfaces of the substrates into contact. The electric field produces an ion transport in the glass substrate and forms a space charge zone between the two substrates. The space charge zone produces a strong attraction of the surfaces of the two substrates, which ensure contact with one another after the approach and thus form a permanent connection. The bonding method is thus based primarily on the maximization of the contact surface of the two surfaces.
Another permanent bonding method is the eutectic bonding. During eutectic bonding, an alloy is produced with a eutectic concentration or is set during the bonding. By exceeding the eutectic temperature, the temperature at which the liquid phase is in equilibrium with the solid phases of the eutectic, the eutectic melts completely. The liquid phase of the eutectic concentration that is produced wets the surface of the areas that are still not liquefied. During the solidification process, the liquid phase solidifies to form the eutectic and forms the connecting layer between the two substrates.
Another permanent bonding method is the fusion bonding. In the case of fusion bonding, two flat, pure substrate surfaces are bonded to one another by making contact. In this case, the bonding process is divided into two steps. In a first step, the two substrates are brought into contact. In this case, the attachment of the two substrates is carried out primarily by van der Waals forces. The attachment is referred to as a prebond. These forces make it possible to produce an attachment, which is strong enough to bond the substrates tightly to one another in such a way that a mutual shifting, in particular by the application of a shearing force, is possible only with a considerable expenditure of energy. In contrast, the two substrates, in particular by applying normal force, can be relatively easily separated from one another again. The normal forces in this case preferably engage on the edge in order to produce a wedging action in the boundary surface of the two substrates, which produces a continuous crack and thus separates the two substrates from one another again. In order to produce a permanent fusion bond, the substrate stacks are subjected to a heat treatment. The heat treatment results in forming covalent connections between the surfaces of the two substrates. Such a permanent bond that is produced is only possible by the use of a correspondingly high force that in most cases accompanies a destruction of the substrates.
The publication U.S. Pat. No. 5,441,776 describes a method for bonding first electrode to a hydrogenated, amorphous silicon layer. This amorphous silicon layer is deposited by deposition processes on the surface of a substrate.
The publication U.S. Pat. No. 7,462,552 B2 shows a method in which a chemical gas phase deposition (Chemical Vapor Deposition, CVD) is used in order to deposit an amorphous silicon layer at the surface of a substrate. The amorphous layer has a thickness of between 0.5 and 10 μm.
In their publication U.S. Pat. No. 7,550,366 B2, Suga et al. report on an accidentally produced amorphous layer, which is approximately 100 nm thick. This amorphous layer is located between the two substrate surfaces, which were prepared by a surface-activation process. The amorphous layer is a by-product of the ion bombardment of the substrate surface with inert gas atoms and metal atoms. The actual bonding process thus takes place between iron atoms, which cover the amorphous layer.
The heat treatment represents another technical problem. The bonded substrates have very often already been provided with functional units, such as, for example, microchips, MEMs, sensors, and LEDs, which have a temperature sensitivity. In particular, microchips have a relatively strong doping. At elevated temperatures, the doping elements have an elevated diffusivity, which can result in an undesirable, disadvantageous distribution of the dopings in the substrate. In addition, heat treatments are always associated with elevated temperatures and thus also with higher costs, with the production of thermal voltages, and with extended process times for heating and cooling. In addition, bonding is to be done at the lowest possible temperatures in order to prevent shifting of different substrate areas, which are comprised of different materials and thus in general also of different thermal expansion coefficients.
A plasma treatment for purification and activation of a substrate surface would be an option for bonding at relatively low temperatures. Such plasma methods do not work or work only very poorly, however, in the case of oxygen-affine surfaces, in particular in the case of metal surfaces. The oxygen-affine metals oxidize and in general form relatively stable oxides. The oxides are in turn an obstacle for the bonding process. Such metals can also be bonded to one another with relative difficulty by diffusion bonding. The bonding of plasma-activated, in particular monocrystalline, silicon, which forms a silicon dioxide layer, works very well, however. The silicon dioxide layer is extremely well suited for bonding. The above-mentioned negative effects of the oxides therefore do not necessarily relate to all classes of materials.
The literature contains several approaches that describe direct bonding at lower temperatures. One approach in PCT/EP2013/064239 includes applying a sacrificial layer, which is dissolved in substrate material during and/or after the bonding process. Another approach in PCT/EP2011/064874 describes the production of a permanent connection by phase conversions. The above-mentioned publications relate in particular to metal surfaces, which are more likely bonded via a metal bond and not via covalent bonds. In PCT/EP2014/056545, an optimized direct bonding process of silicon by a surface purification is described.
The surface roughness of the surfaces/contact surfaces to be bonded represents another problem. In particular, when removing oxides and contaminants from the surfaces of the substrates that are to be bonded with one another with known methods, frequently a higher level of roughness is produced. On the microscopic scale, this roughness prevents full contact between the two surfaces during the bonding process, which has an adverse effect on the effective bonding strength. The two substrate surfaces bond almost overwhelmingly on tangent surface maximum points. Therefore, in particular, a contrast exists between good purification and provision of a surface that is as ideal as possible.
In the semiconductor industry, in particular substances or materials of the same type are to be connected to one another. The type similarity ensures that the same physical and chemical properties are present across the connecting point. This is in particular important for connections, across which electrical current is to be conducted, which are to have a low tendency toward corrosion and/or the same mechanical properties. Among these substances of the same type, primarily the following are found:
Copper-copper
Aluminum-aluminum
Tungsten-tungsten
Silicon-silicon
Silicon oxide-silicon oxide
Some of the metals used in the semiconductor industry are oxygen-affine. Thus, under an oxygen-containing atmosphere, aluminum forms a relatively solid aluminum oxide. During bonding, such oxides have a negative effect on the bonding result, since they are trapped between the two materials that are to be bonded to one another. Under extreme conditions, such an oxide can completely prevent a bonding process; under the most optimal conditions, the oxide is trapped. A mechanical breaking of the oxide layer before the incorporation is also conceivable. The oxide is thermodynamically stable enough not to decompose or to go into solid solution. It remains as oxide in the bonding boundary surface and has a negative effect on the mechanical properties there. Similar problems arise for tungsten and/or copper bonds.